Space Defence Satellites.

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Space Defence Satellites


Satellites (artificial satellites) are ant object that has been purposely placed into orbit around Earth, other planets, or the Sun. Since the launching of the first satellite in 1957, thousands of these “man-made moons” have been rocketed into Earth orbit. Today, satellites play key roles in the communications industry, in military intelligence, and in the scientific study of both Earth and outer space.

Types of Satellites

Engineers have developed many kinds of satellites; each designed to serve a specific purpose or mission.  For instance the telecommunications and broadcasting industries use communications satellites to carry radio, television, and telephone signals over long distances without the need for cables or microwave relays. Navigational satellites pinpoint the location of objects on Earth, while weather satellites help meteorologists forecast the weather. The United States government uses surveillance satellites to monitor military activities.  Scientific satellites serve as space-based platforms for observation of Earth, the other planets, the Sun, comets, and galaxies, and are useful in a wide variety of other applications.

Satellite Launches

Placing a satellite into orbit requires a tremendous amount of energy, which must come from the launch vehicle, or device that launches the satellite. The satellite needs to reach an altitude of at least 120 miles and a speed of over 18,000 mph to lift into orbit successfully. Satellites receive this combination of potential energy (altitude) and kinetic energy (speed) from multistage rockets burning chemical fuels.

The first stage of a multistage rocket consists of rocket engines that provide a huge amount of force, or thrust. The first stage lifts the entire launch vehicle—with its load of fuel, the rocket body, and the satellite—off the launch pad and into the first part of the flight. After its engines use all their fuel, the first stage portion of the rocket separates from the rest of the launch vehicle and falls to Earth. The second stage then ignites, providing the energy necessary to lift the satellite into orbit. It, too, then separates from the satellite and any remaining rocket stages.

The rest of the launch depends on the satellite’s mission. For example, if the mission requires a geostationary orbit, which can be achieved only at a distance of about 22,000 miles above Earth, a third rocket stage provides the thrust to lift the satellite to its final orbital altitude. After the satellite has reached the final altitude, another rocket engine fires and gives the satellite a circular orbit. All rocket-engine burns occur at a precise moment and last for a precise amount of time so that the satellite achieves its proper position in space.

In 1990 the United States began launching some satellites from aircraft flying at high altitudes. This method still requires a rocket-powered launch vehicle, but because the vehicle does not have to overcome friction with the thick atmosphere found at low altitudes, much less fuel is needed. However, the size of the rocket is limited by the size and strength of the aircraft, so only smaller satellites can be launched this way.

Another method of launching satellites is to have astronauts launch them from the U.S. space shuttle. The space shuttle can carry large satellites and, because the shuttle is already in orbit when the satellite is launched, the astronauts can verify that the satellite has survived the rigors of launch. The space shuttle also brings satellites back to Earth for repair.

The Single Stage to Orbit (SSTO) is a launch vehicle that may lower the cost of launching satellites by decreasing the number of launch stages needed and increasing the reusability of launch vehicles. The SSTO would be a piloted vehicle like the space shuttle, but it would be designed to launch satellites more inexpensively and efficiently than the space shuttle can.

Operations in Space

As satellites must survive the launch and must operate in the harsh environment of space, they require unique and durable technologies. Satellites have to carry their own power source because they cannot receive power from Earth. They must remain pointed in a specific direction, or orientation, to accomplish their mission. Satellites need to maintain proper temperature in the face of direct rays from the Sun and in the cold blackness of space. They must also survive high levels of radiation and collisions with micrometeoroids.  Most satellites have onboard computers that help with satellite operations and with the satellite’s mission.

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A satellite provides its own power for the duration of its mission, which can extend to ten years or more. The most common source of power for Earth-orbiting satellites is a combination of solar cells with a battery backup. Solar cells need to be large enough to provide the power that the satellite requires. For example, the solar array of the complex Hubble Space Telescope is about 3,120 sq ft in area and generates about 5,500 watts of electricity, while the solar array of a smaller Global Positioning System satellite is about 50 sq ft in area and ...

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